In present-day gasification processes, the excess residual quench water arising during entrained-flow gasification is taken through one or more stages in the soot water flash-evaporation system from the gasification pressure to a pressure below the ambient pressure, which can be between 2 and 5 MPa (20 and 50 bar), and is therein usually first cooled to approximately 100-110° C. and then to approximately 50-80° C. The soot water under pressure is therein flash-evaporated in the first stage to around ambient pressure by means of special control valves. The resulting flash steam/liquid mixture is separated in a knock-out vessel before the liquid phase is similarly fed to a second flash-evaporation vessel in which expansion into the vacuum region takes place. The liquid phase that is separated off here is mixed during the ensuing soot water conditioning process with flocculant to promote the separating of solids in the downstream thickener/lamella clarifier. The underflow here augmented with solid material (slurry) arrives at another dewatering stage. Clarified effluent from the thickener/lamella clarifier and the filtrate of that dewatering stage (press filtration, for example) are buffered and returned to the gasification process via appropriate high-pressure pumps as circulation water.
Only by means of the described two-stage flash-evaporation operation can the solids-laden soot water be cooled in the necessary manner without making contact with heat transfer surfaces and simultaneously fed without pressure to the robust solids-separating stage employed consisting of a thickener/lamella clarifier and another dewatering stage.
Because the thus cleaned soot water is returned to virtually the same place in the process, the flash-evaporated and cooled water has to be restored to its relevant gasification pressure and, if possible, also heated. Owing to the shifted ratio of calcium to carbonic acid in the flash-evaporated liquid and the pH values >8, this, however, results in the formation of carbonates and hence of bonded deposits and stratifications especially on the heat transfer surfaces. Because the returned soot water is not or is only partially heated, the temperature of the quench water drops, thereby resulting in a reduced water-absorbing capability of the raw gas. This in turn necessitates the addition of more steam upstream of the CO shift system and so leads to higher operating costs.
On the one hand, in order to avoid incrustations/deposits in the heat exchangers, the returned, cleaned soot water is conventionally not heated when 2-stage flash evaporation is employed. The make-up water added to the process is instead heated to the desired quench water temperature. On the other hand, since that does not suffice in many cases, a small amount of high-pressure steam is injected upstream of the CO shift system.